Pd nanoparticles-enabled optical fiber hydrogen sensor with a hydrophobic filter layer for humid conditions

Hydrogen can be stored in underground wells to mitigate the imbalance between hydrogen supply and demand in the future hydrogen economy. The concentration of stored hydrogen can vary due to microbial reactions and leakage through caprocks in the subsurface storage facilities such as salt caverns, saline aquifers, and depleted hydrocarbon reservoirs. Thus, monitoring hydrogen concentration in subsurface storage is essential to ensure integrity of the storage infrastructure and to detect early signs of gas leakage. Optical fiber-based hydrogen monitoring has advantages of stability in harsh environments, real-time and remote sensing, and improved safety compared to electrical-based sensors in flammable gases. An optical fiber sensor with a palladium nanoparticles-incorporated SiO₂ film (Pd/SiO₂) was previously demonstrated for hydrogen sensing over a wide range of hundreds ppm to 100% in dry conditions. However, the Pd-based hydrogen sensitive materials are susceptible to water vapor interference, which leads to a significant reduction in hydrogen sensitivity under humid conditions. To address this challenge, this study focused on the enhancement of hydrogen sensing response under humid conditions by applying a hydrophobic filter film over the Pd/SiO₂ sensing layer. The optical fiber sensor covered by the filter layer showed significant improvement on the baseline drift issue and reduction in hydrogen sensitivity caused by high humidity (99% RH). In addition, the developed optical fiber sensor demonstrated negligible impact by hydrocarbon contaminants such as CO₂ and CH₄ which are present in the subsurface hydrogen storage reservoirs. The Pd/SiO₂-coated optical fiber sensor coupled with the filter layer has high potential to be deployed in the subsurface hydrogen storage areas to monitor hydrogen concentration without cross-sensitivity of hydrocarbons and humidity.